The math concepts in all/any case interviews are basically high-school math concepts. There is no need to spend your hard-earned money on math drills online; save it for the expert interviews if you need those. While I was prepping for mckinsey, I made my own drills and made some mental math a part of my daily routine - while I was on the bus, or on an evening walk. It's very easy, and here are a few tricks to try out:

Ex-MBB, Experienced Hire; Case prep seminar leader; I will teach you not only the how, but also the why of case interviews

PrepLounge actually has very good material (I have no incentive to pitch their product btw). Otherwise, just spend 15-20 minutes a day in Victor Cheng's excellent caseinterviewmath.com. I did this while prepping get for my interviews, it helped me tremendously. It helps do quick math in your head, and see how you compare to others (your competition basically), in terms of both speed and accuracy. I thought I was good until I saw my results. The good news is, a bit of practice did wonders for me - and yes, I did join BCG :)